Recently, an SLD which is between a semiconductor laser and a light emitting diode has been noticed. This light emitting device has a broad spectrum similar to the light emitting diode, and can emit light having high output power similarly to the semiconductor laser. Thus, the SLD has an advantage that light having high output power and low coherency is taken out with good directionality. With this advantage, the SLD has been recently used as a light source for a fiber gyroscope, which is incorporated in an inertial navigation system installed in aircrafts or marine vessels for obtaining position information from one minute to the next, for their destinations.
Important points in the structure and the production of the SLD reside in how to broaden spectral width of spontaneous emitted light as well as how to increase light output power while suppressing laser oscillation.
FIG. 6 is a perspective view illustrating a prior art SLD of a type in which light generated in an excitation region is absorbed in non-excitation region, this type is hereinafter referred to as non-excitation region absorbing type SLD. In the figure, there are successively disposed on an n-type GaAs substrate 1, an n-type AlGaAs cladding layer 2, an undoped AlGaAs active layer 3, a p-type AlGaAs cladding layer 4 and an n-type GaAs cap layer 5. A p-side electrode 6 is disposed on the n-type GaAs cap layer 5 and an n-side electrode 7 is disposed on the substrate 1. A Zn-diffused region 8 for producing a current injected region in the undoped AlGaAs active layer 3, reaches from the n-type GaAs cap layer 5 to the p-type AlGaAs cladding layer 4. This Zn-diffused region 8, as illustrated in FIG. 7, a plan view of SLD in FIG. 6, is disposed perpendicular to a front facet (a) in the element length direction, and the other end opposite to the front facet (a), does not reach the rear facet (b).
Next, a description is given of the operations.
When a forward bias voltage is applied to p-n junction of the SLD, i.e., a positive voltage is applied to the p-side electrode 6 and a negative voltage to the n-side electrode 7, electrons and holes are injected into a region of the active layer 3 directly below the Zn-diffused region 8, radiative recombination electrons and holes occur, and spontaneous and stimulated light is emitted from the facet (a).
As for this non-excitation region absorbing type SLD, FIG. 8 is a diagram illustrating light generated in an excitation region (c) inside the active layer 3. For example, it is supposed that light (c.sub.1), which is generated in the current injected region directly below the Zn diffused region 8 inside the element, i.e., the excitation region (c), having a directionality perpendicular to the rear facet (b), travels in the perpendicular direction (D) and is reflected at the rear surface (b) and return to the excitation region (c). Then, this light (c.sub.1) is synthesized with light (c.sub.2) having a directionality perpendicular to the front facet (a) in the region (c), and the synthesized light is amplified and reaches the front facet (a). This amplified light is again reflected at the facet (a) and returns to the excitation region (c), and is amplified. By repeating these reflections and amplifications, coherent light is generated, resulting in laser oscillation. Once laser oscillation occurs, it is impossible to obtain low coherency light with good directionality. In order to prevent such laser oscillation, the SLD is a non-excitation region absorbing type structure, in which a non-excitation region (d) is produced in a portion from an end of the excitation region (c) opposite to the front facet (a), reaching the rear facet (b) with the stripe shaped Zn diffused region 8 shortened. In this structure, the light generated in the excitation region (c), including the light reflected at the facet (a) and amplified in the excitation region (c), is absorbed in the non-excitation region (d). As illustrated in the figure, while light (A) having a directionality perpendicular to the rear facet (b) travels in the perpendicular direction (B) and is reflected at the rear surface (b), this light (A) is absorbed in the non-excitation region (d), without returning to the excitation region (c). Thus, spontaneous and stimulated light are emitted from the facet (a).
FIG. 9(a) shows a spectrum of light produced by a non-excitation region absorbing type SLD of FIG. 9(b), having a resonator length (l.sub.1) of 500 microns, a Zn diffused region length (l.sub.2) of 250 microns, and a Zn diffused region width (W) of 5 microns.
In the prior art non-excitation region absorbing type SLD constituted as described above, while, in operating at low power light output such as 5 mW, a broad spectrum is obtained as shown in FIG. 9(a), and in operating at high power light output such as 10 mW, the energy amount of light generated by radiative recombinations in the excitation region increases, and the degree or amount of the light which is reflected at the facet (a) and is amplified in the excitation region (c) of FIG. 8 increases. Therefore, even if that light is absorbed in the non-excitation region, it is not absorbed sufficiently, and the reflections and amplifications of the light having directionality perpendicular to the facet are repeated, unfavorably resulting in laser oscillation as shown in FIG. 9(a)
As one that solves this problem, another prior art SLD is disclosed in Japanese Published Patent Application No.3-16186 which is also illustrated in FIG. 10. As shown in FIG. 10, on the surface of a p-type GaAs substrate 21, there are successively grown a p-type Al.sub.x1 Ga.sub.(1-x1) As cladding layer 22, a p-type Al.sub.x2 Ga.sub.(1-x2) As active layer 23, an n-type Al.sub.x1 Ga.sub.(1-x1) As cladding layer 24 and an n-type Al.sub.x3 Ga.sub.(1-x3) As layer 25, by epitaxial growth. Then, about one-third of the element length from the rear facet of these layers are etched away up to reaching the substrate 21. Further, an n-type Al.sub.x4 Ga.sub.(1-x4) As layer 26 having an energy band gap larger than that of the active layer 23 and an n-type GaAs contact layer 27 are laminated by epitaxial growth, and a U-shaped Al.sub.2 0.sub.3 insulating film 28 in a stripe shape, which stripe extents from the front facet in a shorter length than the active layer 23, is produced on the contact layer 27. Then, an electrode 29 and an electrode 30 are respectively disposed on the top and rear surfaces of the element, respectively, and low reflectance coating films 31 and 32 are coated onto the front and rear facets of the device, respectively.
In the above-described construction, when a bias voltage is applied between the upper and lower electrodes 29 and 30, a current is injected into a stripe part 29a whereon the Al.sub.2 0.sub.3 insulating film 28 is not disposed, and light emission starts in the active layer 23 at this region. When a larger current is injected, the device produces gain, generating stimulated emission light. In this construction, since the n-type Al.sub.x4 Ga.sub.(1-x4) As layer 26 is disposed between the active layer and the rear facet of the chip, a light wave traveling toward the rear facet in the active layer 23 is emitted into the semiconductor layer 26 from a rear end of the active layer, and a portion of the emitted light is reflected at the rear facet of the chip and returns to the active layer. In the emission into the semiconductor layer 26, the light is widely diffused, and the portion of the light returning into the active layer is quite small, suppressing laser oscillation even at high power light output operation.
In this prior art SLD, however, production cost is increased, because two epitaxial processes are required, and further, reliability is lowered to a great extent because the regrowth interface is likely to be deteriorated.